Retinal rod outer segments (ROS) inhibit the metabolic activities of cultures of lymphocytes or of other tested cells that grow in suspension. On the other hand, tested cells which attach to the surface of culture containers were not inhibited or were even stimulated by ROS. Preparations of intact ROS discs were less inhibitory than disrupted ones. Yet, disrupted ROS discs do not leak upon incubation any significant inhibitory activity. In addition to vitamin E, other natural antioxidants were found to counteract the inhibitory effect of ROS. These include two enzymes, superoxide dismutase and catalase.